Penetration detection in Nd: YAG laser welding of sheet metal by optical sensors

For the industrial application of laser welding it is important to apply the highest possible welding speed, which is obtained from process windows based on experimental results. For economical reasons one wants to set the parameters as close as possible to the edge of this process window. This might result in insufficient penetration, for instance if the speed is too high. This paper describes the results of two commercially available systems to monitor the welding status. Both systems measure the emissions from the weld pool using optical sensors. The intensity of the emissions in the IR, visible and reflected Nd:YAG part of the spectrum are measured co-axially. The transient response of these sensor signals, to variations in laser power under different welding conditions, has been analyzed. These conditions cover both fully and partially penetrated welds in 0.7 mm mild steel sheets using a Nd:YAG laser. The transition between partially and fully penetrated welds could clearly be monitored.